1. Field of the Invention
This invention relates to a system for detecting a load on a generator.
2. Prior Art
An electricity generating system comprises an engine such as a diesel engine, and a generator connected to the engine. In such an electricity generating system, it is necessary that the frequency of the output of the generator be kept at a predetermined level even when the load connected to the generator varies in magnitude or value.
In one conventional electricity generating system as disclosed in Japanese Utility Model Publication No. 7072/83, the speed of rotation of a generator is detected, and such a detected signal is fed back to a fuel supply control circuit to control the amount of fuel supply to an engine so that the speed of rotation of the generator can coincide with a preselected speed. With this arrangement, when the output frequency of the generator temporarily varies with a variation in the load, the output frequency is returned to a predetermined level. However, this is not sufficient to prevent a variation of the output frequency of the generator which occurs instantaneously immediately after the load varies. For this reason, the system of the above Japanese utility model publication is provided with a device for directly detecting the value of the load.
In this conventional load detecting device, an alternating (ac) voltage signal is obtained from an alternating current (which passes through the load) through a current transformer and a resistor, and this alternating voltage signal is rectified by a rectifier and is smoothed by a ripple filter to produce a dc voltage signal corresponding to the value of the load. In addition to the above detection signal representative of the rotational speed of the generator, this dc voltage signal is also fed back to the fuel supply control circuit to control the amount of fuel supply to the engine in order to prevent an instantaneous variation of the output frequency of the generator which occurs immediately after the load varies in value.
In the above load detecting device, however, since the alternating voltage signal is smoothed by the ripple filter, the variation of the dc voltage signal corresponding to the variation in the load appears considerably after the load is subjected to such a variation. Such time delay can not be disregarded. Thus, it has been difficult to restrain the instantaneous variation of the above output frequency to a satisfactory level.
In an attempt to overcome this difficulty, a load detecting device has been proposed in Japanese Laid-Open (Kokai) Patent Application No. 135396/86. More specifically, an alternating voltage signal is obtained as in the device of the above Japanese utility model publication. The alternating voltage signal is rectified by a full-wave rectifier circuit so as to produce a waveform voltage signal. This conventional device further comprises a sample-and-hold circuit and a rotation detecting device serving as a sampling instruction means. The rotation detecting device outputs pulses in accordance with the rotation of an engine. The pulse is outputted each time the waveform voltage signal is at its peak. These pulses outputted during such a short time period serve as a sampling instruction signal. The sample-and-hold circuit is responsive to the sampling instruction signal to effect the sampling of the peak voltage of the waveform voltage signal and to hold it. The thus obtained peak voltage is fed back as a signal representative of the value of the load.
With this conventional load detecting device, the peak voltage of the waveform voltage signal is detected without using a ripple filter, and therefore variations in the load can be detected without undue delay. Thus, the instantaneous variation of the output frequency of the generator can be restrained theoretically.
In the load detecting device of the above Japanese Laid-Open Patent Application No. 135396/86, the rotation detecting device is required t be connected to the output shaft of the engine with high precision so that the sampling instruction signal can be outputted at the time when the waveform voltage signal is at its peak. However, this is rather difficult. Therefore, actually, the timing of outputting the sampling instruction signal does not coincide with the time when the waveform voltage signal is at its peak. As a result, the variation in the load could not be accurately detected, thus failing to positively restrain the instantaneous variation in the output frequency of the generator.